Learn English – Inversion by “only” in a passive sentence

The idea of "converting" an active to a passive and vice versa is something of a misnomer. You can create pairs of active/passive sentences that are approximately equal in meaning, but probably never exactly equivalent.

One reason for this has to do with information theory, in other words, the types of things/information that we tend to "encode" in particular syntactic positions in a sentence. In the sentence "I gave Dave the book", I am focussing slightly on my action of giving compared to "Dave was given the book", where I focus slightly on the impact of David receiving the book. This means that it is odd to "convert" e.g. "The bus left the station" into "The station was left by the bus", because we don't conceptualise the movement of a bus from a station as having an "impact" on the station. (Other cases where the passive would be odd include e.g. "A headache was given to Jane", "The prime minister was become by David Cameron", "10 is equalled by 5+5"...)

Another reason has to do with scope effects. If I say "Chocolate is eaten by children", this tends to imply that (a) children are the only/principal entities to eat chocolate and (b) children may eat a range of other things. But if I say "Children eat chocolate", this tends to imply that (a) children primarily eat chocolate over other food, and (b) does not imply that children are the only/main entities to eat chocolate.

These can be analysed either as instances of Subject-dependent inversion, or as Subject-postposing. One reason for choosing the latter is that postposing of the Subject is entirely optional. Notice that in the Original Poster's examples the two instances of so are quite different. They are both used as anaphoric pro-forms, but are being used to represent quite different types of phrase or clause which both have very different grammatical functions:

1. So says the preacher.

2. So began the fight.

In the first sentence so is the Complement of the verb say and represents a content clause. In (2) so is an Adjunct meaning something like in this way. When anaphoric so is preposed like this, we can also postpose the Subject. This is optional as can be seen from the examples below:

• So the preacher says.
• So the fight began.

(Notice that so must be stressed in the last example - probably to distinguish this usage from the usage of so as a connective or conjunction.)

The grammatical status of the proform so makes a big difference to the effect of postposing the Subject. When so is the pro-clause Complement of the verb say, the Subject postposing is quite normal. However, fronting of so used as a manner adjunct gives quite a literary effect: So endeth the lesson, So began my first term and so forth.

So is not a conjunction or conventional discourse connective in either of the Original Poster's sentences - although the word so can be used as a connective as is illustrated below:

• He took my ice-cream, so I punched him on the nose.